1. Field of the Invention
The invention relates to a process and device for use in flash smelting sulphide ores or concentrates.
2. Description of the Prior Art
In the currently used oxidation-reduction process, pyrite is suspended in hot, oxygenous smoke gases at the upper end of the reaction shaft of a flash smelting furnace. Both a thermal decomposition of the pyrite and a simultaneous partial oxidation of the decomposed sulphur and the iron matte produced in the decomposition process take place in the suspension. The hot, oxygenous smoke gases are obtained by burning oil with a high air coefficient.
The products of the reactions in the reaction shaft are a gas which contains the following compounds, among others: S.sub.2, SO.sub.2, H.sub.2 S, COS, H.sub.2 and CO, and a melt which consists of FeS, iron oxides and slag. The sulphur content of the gas is produced in the form of elemental sulphur, the melt is granulated and roasted into gaseous sulphur dioxide and iron ore.
The sulphur content in the melt (the rate of iron oxides) is dependent on the oxygen content in the smoke gases fed into the reaction shaft, which again can be regulated by regulating the air coefficient of the oil burning process. By increasing the air coefficient (by decreasing the rate of oil) a larger part of the sulphur present in the concentrate can be released from the concentrate and directed into the gas.
Owing to the more oxidating reaction shaft operation, the rate of sulphur dioxide increases and those of the reducing components (H.sub.2 S, COS, H.sub.2, CO) decrease. The optimal recovery of sulphur prerequires that the gas composition realizes the following equation: EQU SO.sub.2 = 1/2 (H.sub.2 S + COS + H.sub.2 + CO)
for this reason the excess SO.sub.2 has been reduced with light petroleum in the rising shaft of the flash smelting furnace.
The sulphur content in the produced iron matte can be lowered by raising the oxygen content in the gas to be fed into the reaction shaft. The ratio between the production of elemental sulphur and that of gaseous sulphur dioxide can thus be regulated by the reaction shaft oil feed -- the air rate being constant. The rate of oil used in the reaction shaft burners does not have a significant effect on the smelting capacity of the system (FIG. 2).
The air coefficient of the oil burning also effects the fuel consumption in the process. When the operation takes place at the optimal point in regard to the sulphur yield, all the oxygen fed into the reaction shaft in the combustion air must become bound either to the iron removed along with the iron matte or to the carbon and hydrogen of the fuel and the reduction agent. When the air coefficient rises and the sulphur content of the iron matte lowers, the oxygen content of the matte increases and the sum of the requisite oil and reduction petroleum decreases.
In this known oxidation-reduction process it is not possible to efficiently use in the process the heat of combustion of the concentrate. The heat generated in the roasting of the iron matte is produced in the form of high-pressure steam. The rate of air used in the process is high because air is used both at the smelting and the roasting stages.
A decisive improvement is achieved when a so-called sulphur circulation process is adopted in which the iron matte obtained from the flash smelting furnace is roasted either in its entirety or partially in a roasting furnace, from which all the roasting gases are fed, uncooled, into the flash smelting furnace for the smelting of fresh concentrate.
The following advantages are gained in the process:
The consumption of the reduction agent and/or fuel decreases. This is because the rate of oxygen coming into the flash smelting furnace is lower since part of the oxygen becomes bound to iron in the roasting furnace.
The smelting capacity of the flash smelting furnace is mainly dependent on the content of free oxygen in the gas used and on the temperature of the gas. A rise in the temperature and the oxygen content increases the smelting capacity of the flash smelting furnace.
It is known that the roasting capacity of a roasting furnace can be increased by cooling the fluidized bed by means of cooling devices. The cooling of the bed results in a reduction of excess air in the roasting furnace and a reduction of the content of free oxygen in the roasting gas. The roasting capacity of a roasting furnace at a constant roasting temperature and with a constant air rate can be reduced further, and the oxygen content in the roasting gas can be increased by pre-heating the roasting air.
When in the present process the gas obtained from the roasting furnace is used in the flash smelting furnace for smelting pyrite, the low rate of free oxygen in the gas has a decreasing effect on the flash smelting furnace capacity. A high gas temperature again increases the smelting capacity in comparison to cold air. With the joint effect of these two and by using an uncooled roasting furnace, a smelting capacity which is approximately the same as when using cold air is obtained in the flash smelting furnace.
It is obvious from the above that in this process the capacities of the smelting furnace and the roasting furnace can be controlled by means of cooling devices placed in the fluidized bed in the roasting furnace or by pre-heating the roasting air. Cooling the bed increases the roasting capacity of the roasting furnace and decreases the smelting capacity of the flash smelting furnace. Pre-heating the roasting air produces the opposite effect.
By choosing an appropriate degree of cooling the roasting furnace, the capacities of the roasting furnace and the flash smelting furnace can be balanced so that the rate of melt produced by the flash smelting furnace corresponds to the capacity of the roasting furnace. In this case all the sulphur present in the concentrate is obtained in the form of elemental sulphur. By lowering the degree of cooling of the roasting furnace or by further pre-heating the roasting air, the desired proportion of the iron matte produced in the flash smelting furnace is left for roasting in another roasting furnace to produce gaseous sulphur dioxide (FIGS. 3 and 4).
The capacity of the flash smelting furnace can be raised without significantly affecting the capacity of the roasting furnace, by enriching the roasting air or the roasting gas with oxygen. In this case a higher degree of cooling is required for the roasting furnace in order that the roasting furnace capacity correspond to the iron matte output of the flash smelting furnace. The correspondence is achieved with a greater smelting capacity (FIG. 5).
When copper or nickel concentrate is used as feed in the oxidation-reduction process, the excess oxygen in the reaction shaft must be sufficient, because the slagging of iron at the smelting stage requires a high oxygen pressure in comparison to the pyrite process. When copper concentrate is used in the process according to the invention, the "treatment" of the copper matte from the flash smelting furnace takes place in a previously known manner in a copper converter, the gases of which are then fed as such or concentrated in regard to SO.sub.2, possibly mixed with air, into the smelting stage of the flash smelting furnace. With this procedure, the entire sulphur content of the copper concentrate is recovered as elemental sulphur.
Even in the sulphur circulation process the energy is used disadvantageously, because the gas emerges hot from the flash smelting furnace. Especially when the aim is to produce the entire sulphur content of the pyrite as elemental sulphur, the capacity of the process remains low (FIGS. 3 and 4, operation points with no SO.sub.2 production).
Raising the capacity by means of oxygen is not the best method, owing to operation and investment costs.
An economic alternative is to use the heat content of the hot gases emerging from the flash smelting furnace for increasing the capacity. However, it is not advantageous to bring energy into the roasting furnace by pre-heating the combustion air, because it has the effect described above on the ratio between the elemental sulphur and the gaseous SO.sub.2 produced from the excess iron matte. Furthermore, the use of the exhaust gases for pre-heating the air is technically difficult because of the molten dust present in the gases.
Both in the sulphur circulation process and the currently used oxidation-reduction process it is possible to pre-heat the concentrate used, e.g., pyrite, but owing to the low heat capacity of the concentrate the obtained benefit is insignificant and this method is not widely used.